The invention relates to a method in a dryer section provided with a single-wire draw in a paper machine or a paper finishing machine, wherein a paper web to be dried is passed on support of a drying wire alternatingly over heated cylinder faces of drying cylinders and over leading rolls or equivalent. On the drying cylinders, the paper web is in direct contact against the heated cylinder face and, on the leading rolls, the paper web is placed on the outside face of the drying-wire loop at the side of the outside curve.
Further, the invention relates to a wire group in a multi-cylinder dryer in a paper machine, comprising one or several smooth-faced and solid-mantle heated drying cylinders and leading rolls or equivalent drying cylinders placed at in proximity thereto. The wire group comprises a drying-wire loop guided by guide rolls and arranged to be curved over the drying cylinders and leading rolls so that the drying cylinders are placed outside the drying-wire loop and the leading rolls are placed inside the drying-wire loop. The drying wire is fitted to press the paper web to be dried against the heated cylinder faces of the drying cylinders, while the paper web is fitted to curve on the outside face of the drying wire over the leading rolls when the paper web runs from a preceding drying cylinder onto a subsequent drying cylinder.
In multi-cylinder dryers of a paper machine, during the passing of the paper web through the paper machine, either a so-called twin-wire draw and/or a single-wire draw is/are used. In a twin-wire draw, the heated drying cylinders are arranged in two horizontal rows placed one above the other, the successive cylinders in the rows being placed in an interlocking relationship, i.e., interlocked, in the upper and lower rows. In such a case, in each cylinder group, there are two drying wires, a so-called upper wire and a lower wire, by whose means the paper web is pressed against heated drying-cylinder faces. The upper and lower wires are guided by guide rolls placed in the gaps between the cylinders. In a twin-wire draw, the web has usually free, unsupported draws as it runs and meanders between the rows of cylinders.
Recently, in dryers, a single-wire draw has become more common, wherein only one drying wire is employed in a cylinder group. The paper web runs through the whole group on support of this single wire. In earlier embodiments of paper machines employing a single-wire draw, two rows of drying cylinders were commonly employed, one row being placed above the other. However, at present, only one row of drying cylinders is employed so that the other row generally consists of unheated leading rolls or cylinders. The drying cylinders, leading rolls, and the drying wire are arranged so that the drying wire presses the web to be dried against the cylinder face and, on the leading rolls, the web is placed at the side of the outside curve. The leading rolls are usually placed inside the drying-wire loop.
In dryer sections provided with a single-wire draw, the leading rolls are typically suction rolls, and preferably suction rolls provided with a grooved outer mantle, marketed by the assignee under the trade mark "VACROLL". By means of the suction effect in these suction rolls, the adhesion of the paper web to the outer face of the drying wire on the reversing sectors of the suction-leading rolls is promoted.
When compared with drying sections having a twin-wire draw, it is a substantial advantage of a single-wire draw that free, unsupported draws of the paper web can be avoided. For this reason, a single-wire draw is commonly employed, in particular, near the forward end of the dryer section where the web has a higher moisture content and is, thus, of lower strength and more susceptible of breaks and stretching.
Recently, a type of dryer section which has also become more common, is one in which exclusively drying groups with a single-wire draw are employed. In such embodiments, it is possible to use either so-called normal groups, in which the drying cylinders are placed in the upper row and the leading rolls in the lower row, or it is additionally possible to employ some so-called inverted groups, in which the cylinders and the leading rolls are placed one above the other in the reversed sequence, i.e., drying cylinders in the lower row and leading rolls in the upper row. In a typical dryer section provided with only a single-wire draw, there are, e.g., about seven successive wire groups.
In dryer sections having a single-wire draw, above all in the wire groups arranged towards the rear end of the dryer section, glazing and wearing in the face of the drying wire placed at the side of the paper has been noticed to be stronger than average. It has also been noticed that the reason for this increase is the speed difference between the wire and the paper web arising in single-wire draw. This speed difference always arises in situations in which the paper web is alternately placed outside the wire and between the wire and the roll as it runs on the face of the same drying wire or felt. The manner in which the difference in speed arises will be described in more detail later with reference to the figures in the accompanying drawing.
It is significant problem that the difference in speed between the paper and the wire produces a sort of "grinding" between them which abrades the surface portion of the wire placed at the side of the paper. This effect may also produce detrimental changes in the quality properties of the paper face.
In the initial end of the dryer section, where the paper web has a higher moisture content and is therefore more elastic, the deformations of the paper web are usually capable of compensating for the effect caused by the difference in speed between the wire and the paper. However, when the paper runs further in the dryer section and becomes dry, its elasticity becomes lower at the same time as the paper web shrinks to some extent. For example, in a dryer section such as mentioned above, e.g., one provided with seven single-wire groups, the dry solids content at the beginning of group 6 is in a range from about 65% to about 70% and at the end of the dryer section, the dry solids content is in a range of from about 90% to about 98%. In this range of dry solids content, the elasticity of the paper is no longer sufficient to compensate for the above drawbacks arising from the differences in speed between the drying wire and the paper web.
The afore-mentioned problems related to the increased wear of the wire to a level considerably higher than average, usually increase steeply and become a significant drawback when the dry solids content of the paper web is higher than from about 60% to about 65%.
The differences in speed between the wire and the paper web may also cause problems of other sorts, such as dust formation in the paper and, with some paper grades, also other detrimental factors, mainly related to the quality of the paper. Also, the tightening of a relatively dry web inside a wire group causes problems in the runnability of the web, e.g., in the form of web breaks. Such web breaks may cause undesirable standstills.
As will be described later in connection with an explanation of the formulae of calculation set forth below and from the exemplifying illustrations, it is possible to reduce the difference in speed between the paper to be dried and the supporting drying wire by making the drying wire thinner. This aspect has also been experimented with in some places to reduce the problems described above. However, in the manufacture of a sufficiently thin drying wire, considerable difficulties are encountered, because the manufacture and operation of a wire of a thickness of, for example, about 1.1 mm is already "fine art", whereas the manufacture and operation of a drying wire of a thickness of 1.5 mm is fully conventional.